Wavelength converting material for a light emitting device

ABSTRACT

Embodiments of the invention include a wavelength-converting material defined by AE 3−x1−y+z RE 3−x2+y−z [Si 9−w Al w (N 1−y C y ) [4] (N 16−z−w O z+w ) [2] ]:Eu x1 ,Ce x2 , where AE=Ca, Sr, Ba; RE=Y, Lu, La, Sc; 0≤x1≤0.18; 0≤x2≤0.2; x1+x2&gt;0; 0≤y≤1; 0≤z≤3; 0≤w≤3.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/582,042 filed Apr. 28, 2017 and entitled “WAVELENGTH CONVERTINGMATERIAL FOR A LIGHT EMITTING DEVICE”, which claims priority to EuropeanPatent Application No. 16168015.2 filed May 3, 2016. U.S. patentapplication Ser. No. 15/582,042 and European Patent Application No.16168015.2 are incorporated herein by reference.

BACKGROUND

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials. Typically, III-nitride light emitting devices are fabricatedby epitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a sapphire, silicon carbide,III-nitride, or other suitable substrate by metal-organic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxialtechniques. The stack often includes one or more n-type layers dopedwith, for example, Si, formed over the substrate, one or more lightemitting layers in an active region formed over the n-type layer orlayers, and one or more p-type layers doped with, for example, Mg,formed over the active region. Electrical contacts are formed on the n-and p-type regions.

A light emitting device such as an LED is often combined with awavelength converting material such as a phosphor. US 2010/0289044describes desirable properties of a red-emitting phosphor. Inparticular, “for >200 lm/W white down-conversion LEDs, red is the mostcritical spectral component since the spectral position and width of thered emission directly determines luminous efficacy and color rendition.Besides high efficiency and stability, a suitable Eu²⁺ doped hostlattice for narrow emission red should fulfill at least part of thefollowing requirements:

“1. Strong, covalent activator-ligand interactions are needed toefficiently lower the net positive charge of the activator. A mediumcondensed nitride lattice with coordinating N[2] ligands is consideredas most suitable.

“2. The host should contain only one substitutional lattice site for theactivator ion and no statistical site occupation within the hoststructure (as found for SiAlONes or CaSiAlN₃:Eu) to avoid inhomogeneousbroadening of the emission band. In case that more than onesubstitutional lattice is present in the host lattice, thesubstitutional lattice sites should differ significantly in chemicalnature to avoid spectral overlap of emission bands.

“3. The activator site should show a high symmetry to limit possiblestructural relaxation modes of the activator in the excited state.Preferably, the activator site is larger (Ba site) than Eu²⁺ to hinderexcited state relaxation and thus minimize the Stokes shift.

US 2010/0289044 further states “Preferably, the red emitting Eu(II)phosphor should also show coordination numbers of the Eu(II) activatorbetween 6 and 8 and an activator ligand arrangement that leads to astrong splitting of the Eu(II) 5d levels required for red emission incombination with a small Stokes shift. The activator-ligand contactlength should lie in the range 210-320 pm. IN other words, a suitablered phosphor is characterized by a six fold to eightfold coordination ofthe red emitting activator by its ligands and activator-ligand contactlengths in the 210-320 pm range.”

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the crystal structure of materials according to someembodiments of the invention.

FIG. 2 is a cross sectional view of an LED.

FIG. 3 is a cross sectional view of a device with a wavelengthconverting structure in direct contact with an LED.

FIG. 4 is a cross sectional view of a device with a wavelengthconverting structure in close proximity to an LED.

FIG. 5 is a cross sectional view of a device with a wavelengthconverting structure spaced apart from an LED.

DETAILED DESCRIPTION

Some embodiments of the invention include luminescent host latticematerials with properties that are suitable for solid state lightingapplications. Embodiments of the invention include awavelength-converting composition as defined by the following formula:AE_(3−x1−y+z)RE_(3−x2+y−z)[Si_(9−w)Al_(w)(N_(1−y)C_(y))^([4])(N_(16−z−w)O_(z+w))^([2])]:EU_(x1),Ce_(x2),where AE=Ca, Sr, Ba; RE=Y, Lu, La, Sc; 0≤x1≤0.18; 0≤x2≤0.2; x1+x2>0;0≤y≤1; 0≤z≤3; 0≤w≤3.

Wavelength-converting compositions according to some embodiments arecharacterized by a cubic crystal structure, a cubic coordination of oneEu²⁺ dopant site by X^([2]) (X═N, O) atoms, and star-shaped Y(SiX₄)₄(Y═C, N) host lattice building blocks.

FIG. 1 illustrates the crystal structure of some embodiments of theinvention. In FIG. 1, structure 100 is an X atom that connects (Si,Al)N₄tetrahedra of the host lattice. Structure 102 is a (Si,Al)N₄tetrahedron. Structure 104 is an X atom that coordinates the centralatom position that can be occupied by AE and RE atoms. Structure 106 isthe central atom position that can be occupied by AE and RE atoms. Eightof the 104-type atoms form a cubic arrangement around 106-type atoms.

In some embodiments, the large Eu(II) dopant incorporates into thelattice at the AE site (Wyckoff position 3d) that is coordinated by acube formed by N^([2]) ligands. The Eu—N distance, which determines theposition of absorption and emission band positions in the blue and redspectral range respectively, increases with the size of AE and RE atoms.

For larger AE cations, such as, for example, Ba, formation of unwantedsecondary phases of composition AE_(3+a)RE_(1−a)Si₆O_(a)N_(11−a) becomesmore likely, for example because of the higher average volume percation. In particular, for example, Ba_(4−x)Ca_(x)Si₆N₁₀O has a volume69.8-70.5 Å³/cation; BaEu(Ba_(0.5)Eu_(0.5))YbSi₆N₁₁ has a volume 71.1Å³/cation; and Ca₃RE₃[Si₉N₁₇] (RE=Sm, Yb) has a volume 64.9-67.4Å³/cation.

In some embodiments, to avoid excessive unwanted secondary phaseformation, the average effective ionic radii (for six fold coordination)for the AE+RE cations should not exceed 120 pm in some embodiments, notexceed 115 pm in some embodiments, and not exceed 110 pm in someembodiments. Examples for such compositions areCa_(2.985)Y₃Si₉N₁₇:Eu_(0.015) (avg. radius˜109 pm),Ca_(1.985)La₄Si₉N₁₆C:Eu_(0.015) (avg. radius˜116 pm),Sr_(1.985)Y₄Si₉N₁₆C:Eu_(0.015) (avg. radius˜113 pm),Sr_(2.98)Sc₃Si₉N₁₇:Eu_(0.02) (avg. radius˜110 pm),Sr_(2.98)Lu₃Si₉N₁₇:Eu_(0.02) (avg. radius˜110 pm),Ca_(5.97)Si₉O₃N₁₄:Eu_(0.03) (avg. radius˜114 pm). Small amounts ofdoping cations such as Eu(II) or Ce(III) do not have an appreciableimpact on relative phase stability.

In some embodiments, an increase of the average cation size leads to ashift of absorption and emission bands towards larger energies. Thenephelauxetic effect of the N^([2]) ligands leading to the low energyabsorption and emission bands in the blue to red spectral range iscomparable to that observed for other commercially available Eu²⁺nitride phosphors (Table 1). As illustrated in Table 1, red Eu²⁺emission can be obtained in very different host lattice environments.The number of coordinating ligands, the contact length, and the electrondensity of the ligand influences the relative position of absorption andemission bands. The number of coordinating ligands is the number ofnearest neighbor atoms being either N or O around the atom site hathosts the luminescent atom (Eu in some embodiments). The contact lengthor bond length is the distance between central atom and ligand. Inparticular, long contact lengths, a high coordination number, and a lowinductive effect of the 2^(nd) coordination sphere cations as present inSrLiAl₃N₄ are chemically comparable with short lengths, a lowcoordination number and a high inductive effect of the 2^(nd)coordination sphere cations as present in e.g. CaSiAlN₃. Besides theactivator site chemistry of the host lattice system according to someembodiments, the high symmetry of the AE site makes the materialaccording to some embodiments especially suitable for narrow bandemission.

TABLE 1 Eu(II) emission sites in various nitride phosphors Avg. radiusCharge of of 2nd coord. N Distance ligand 2nd coord. sphere atoms Hostlattice Site ligand (Angstr.) bonded to sphere atoms (pm) Sr₂Si₅N₈ Sr1N5 2.5704 2 × Si −8 40 N2 2.6266 2 × Si −8 40 N2 2.6266 2 × Si −8 40 N42.8611 3 × Si −12 40 N1 2.8908 2 × Si −8 40 N1 2.8908 2 × Si −8 40 Sr2N1 2.5418 2 × Si −8 40 N2 2.7199 2 × Si −8 40 N2 2.7199 2 × Si −8 40 N52.8945 2 × Si −8 40 N5 2.8945 2 × Si −8 40 N3 2.9594 3 × Si −12 40 N32.9594 3 × Si −12 40 CaSiAlN₃ Ca1 N1 2.407 3 × −10.5 46.5 Al0.5Si0.5 N12.407 3 × −10.5 46.5 A10.5Si0.5 N2 2.45 2 × −7 46.5 Al0.5Si0.5 N2 2.5252 × −7 46.5 Al0.5Si0.5 N2 2.678 2 × −7 46.5 Al0.5Si0.5 SrLiAl₃N₄ Sr1 N52.6721 3 × Al, −10 58 1 × Li N1 2.706 4 × Al, −10 58 1 × Li N8 2.7174 5× Al, −10 58 1 × Li N4 2.7179 6 × Al, −10 58 1 × Li N5 2.871 7 × A1, −1058 1 × Li N1 2.9087 8 × Al, −10 58 1 × Li N3 2.9122 9 × Al, −10 58 1 ×Li N6 2.9185 10 × Al, -10 58 1 × Li Sr2 N2 2.5894 11 × Al, −10 58 1 × LiN7 2.6904 12 × Al, −10 58 1 × Li N8 2.7065 13 × Al, −10 58 1 × Li N42.739 14 × Al, −10 58 1 × Li N2 2.793 15 × Al, −10 58 1 × Li N6 2.854816 × Al, −10 58 1 × Li N3 2.8737 17 × Al, −10 58 1 × Li N7 2.9049 18 ×Al, −10 58 1 × Li AE₃RE₃Si₉N₁₇ AE2 8XN1 2.58-2.7 2 × Si −8 40

Concurrent replacement of the 4-fold connecting nitrogen atom in the[N^([4])(SiN^([2]) ₃)₄] units by carbon according to [C^([4])(SiN^([2])₃)₄] (such that 0<y≤1) and a divalent AE atom by a trivalent RE atom mayincrease the charge density of the lattice and thus the overallstability. One example of such a material isCa_(1.985)La₄Si₉N₁₆C:Eu_(0.015). The partial or full replacement ofN^([4]) by C^([4]) may contribute to suppression of the unwantedincorporation of Eu²⁺ activator ions on the six fold coordinated splitposition (site 6g). Partial replacement of the N^([4]) by carbon mayincrease the charge on this site while a redistribution of charges onthe two metal sites may lead to a more stable host structure with bondvalence sums closer to the theoretical values (value closer to 3 for the6-fold coordinated site and a value closer to 2 for the 8-foldcoordinated site). Examples of such an optimized structure includeCa_(3−x1−y)RE_(3+y)Si₉N_(1−y)C_(y)N₁₆:EU_(x1) with 0.004<=x1<=0.09 and0<y<=1 and Ca_(2.49)La_(0.5)Y₃Si₉N_(16.5)C_(0.5):Eu_(0.01).

Since most of the nitrogen atoms are two-fold connecting with respect tosilicon atoms (N^([2])) it is possible to replace them with oxygen (suchthat z+w>0) as observed, for example, for phases Ba_(4−x)Ca_(x)Si₆N₁₀O.Similar to the charge compensation discussed above, for every Oincorporated on a N site, a trivalent RE atom can be replaced by adivalent AE atom. One example of such a material isCa_(5.97)Si₉O₃N₁₄:Eu_(0.03). Alternatively, charge introduced by Oincorporation can be compensated by SiAlON formation, a formalreplacement of a (Si,N) unit by an isoelectric (Al,O) unit. One exampleof such a material is Ca_(2.985)Y₃Si₈AlON₁₆:Eu_(0.015).

For the synthesis ofAE_(3−x1−y+z)RE_(3−x2+y−z)[Si_(9−w)Al_(w)(N_(1−y)C_(y))^([4])(N_(16−z−w)O_(z+w))^([2])]:Eu_(x1),Ce_(x2)materials according to some embodiments, any suitable precursors may beused and a variety of precursor materials are suitable. Nitrides,hydrides, silicides may for example be used. For incorporation ofalkaline earth elements, AE₃N₂ or AEH₂ compounds are e.g. suitable. Forincorporation of rare earth elements including e.g. yttrium andscandium, silicides RESi₂, Tris[N,N-bis(trimethylsilyl)amide]RE(III) ornitrides REN are suitable. Mixed silicides crystallizing in the ThSi₂structure type of composition AE_(1−x)RE_(x)Si₂ are also suitable. Othersilicon sources are e.g. Si₃N₄, perhydropolysilazane, silicon diimide,silicon or silicon carbide. Suitable dopant compounds are e.g. Eu₂O₃ orEu₂Si₅N₈, CeO₄, CeF₃, or e.g. Tris[N,N-bis(trimethylsilyl)amide]cerium.

The precursor materials can be mixed by e.g. ball milling to obtain amacroscopically homogenous power mass that is then fired under inert orreducing atmosphere. The process can for example consist of a firstfiring under argon atmosphere to form for example an intermetallicprecursor that is then further processed under a nitrogen orhydrogen-nitrogen mixture atmosphere to form the desired nitridephosphor material. A final firing under an elevated nitrogen pressure infor example the range of 1-50 MPa is especially suitable to increase thecrystallinity of the phosphor.

One advantage of wavelength-converting compositions according to someembodiments, as compared to known narrow band emitting materials, is ahigh degree of lattice condensation leading to a large band gap and highstability, and wide compositional tunability that allows fine tuning ofthe luminescence properties.

In some embodiments, an optically isotropic structure renders thematerial system according to some embodiments especially useful forformation into a ceramic phosphor, for example due to superior lightpropagation properties. The luminescent material according to someembodiments is especially suitable for the synthesis of a monolithicceramic material that can be applied to effectively convert blue LEDpump light into long wavelength light. The term “ceramic” hereespecially relates to an inorganic material that is obtainable byheating a (polycrystalline) powder at, for example, at least 500° C. insome embodiments, at least 800° C. in some embodiments, and at least1000° C. in some embodiments, under high pressure, for example at least0.5 MPa in some embodiments, at least 1 MPa in some embodiments, 1 toabout 500 MPa in some embodiments, at least 5 MPa in some embodiments,and at least 10 MPa in some embodiments, for example under uniaxial orisostatic pressure.

In some embodiments, a ceramic is formed by hot isostatic pressing(HIP), in particular a post-sinter HIP, capsule HIP or combinedsinter-HIP process, such as under the temperature and pressureconditions as described above. The ceramic obtainable by such method maybe used as such, or may be further processed (such as by polishing, oreven processing into particles again).

In some embodiments, ceramic including wavelength converting materialsaccording to some embodiments may have a density that is at least 90% insome embodiments, at least 95% in some embodiments, and in the range of97-100% in some embodiments, of the theoretical density (i.e. thedensity of a single crystal). In some embodiments, ceramic includingwavelength converting materials according to some embodiments may bepolycrystalline, but with a reduced, or strongly reduced volume betweengrains (pressed particles or pressed agglomerate particles).

In some embodiments, uniaxial or isostatic pressure may be applied toobtain the phosphor. Some embodiments of the invention include a methodfor producing the above-described wavelength converting materials byselecting starting materials in ratios that can lead to at least thedesired phosphor and heating under pressure, especially uniaxial orisostatic pressure, even more especially isostatic pressure, thestarting materials to produce at least the desired wavelength convertingmaterials. The wavelength converting material may be formed at, forexample, at least 1400° C. in some embodiments, up to about 1800° C. insome embodiments, and pressures from atmospheric pressure up to theabove indicated pressures or even above in some embodiments.

In some embodiments, a ceramic including a wavelength convertingmaterial according to some embodiments as described above may alsoinclude (a) one or more other type of phosphors, (b) one or more otherphases formed during synthesis of the one or more of the hereindescribed wavelength converting materials (respectively), (c) one ormore starting materials used during synthesis of the one or more of theherein described wavelength converting materials (respectively), (d) oneor more fluxes used during synthesis of the one or more of the hereindescribed wavelength converting materials (respectively), (e) one ormore scattering materials, and (f) one or more other materials (such asa halide salt).

The wavelength converting materials described above may be used, forexample, in a light source including a light emitting diode. Lightemitted by the light emitting diode is absorbed by the wavelengthconverting material according to embodiments of the invention andemitted at a different wavelength. FIG. 2 illustrates one example of asuitable light emitting diode, a III-nitride LED that emits blue light.

Though in the example below the semiconductor light emitting device is aIII-nitride LED that emits blue or UV light, semiconductor lightemitting devices besides LEDs such as laser diodes and semiconductorlight emitting devices made from other materials systems such as otherIII-V materials, III-phosphide, III-arsenide, II-VI materials, ZnO, orSi-based materials may be used.

FIG. 2 illustrates a III-nitride LED 1 that may be used in embodimentsof the present invention. Any suitable semiconductor light emittingdevice may be used and embodiments of the invention are not limited tothe device illustrated in FIG. 2. The device of FIG. 2 is formed bygrowing a III-nitride semiconductor structure on a growth substrate 10as is known in the art. The growth substrate is often sapphire but maybe any suitable substrate such as, for example, SiC, Si, GaN, or acomposite substrate. A surface of the growth substrate on which theIII-nitride semiconductor structure is grown may be patterned,roughened, or textured before growth, which may improve light extractionfrom the device. A surface of the growth substrate opposite the growthsurface (i.e. the surface through which a majority of light is extractedin a flip chip configuration) may be patterned, roughened or texturedbefore or after growth, which may improve light extraction from thedevice.

The semiconductor structure includes a light emitting or active regionsandwiched between n- and p-type regions. An n-type region 16 may begrown first and may include multiple layers of different compositionsand dopant concentration including, for example, preparation layers suchas buffer layers or nucleation layers, and/or layers designed tofacilitate removal of the growth substrate, which may be n-type or notintentionally doped, and n- or even p-type device layers designed forparticular optical, material, or electrical properties desirable for thelight emitting region to efficiently emit light. A light emitting oractive region 18 is grown over the n-type region. Examples of suitablelight emitting regions include a single thick or thin light emittinglayer, or a multiple quantum well light emitting region includingmultiple thin or thick light emitting layers separated by barrierlayers. A p-type region 20 may then be grown over the light emittingregion. Like the n-type region, the p-type region may include multiplelayers of different composition, thickness, and dopant concentration,including layers that are not intentionally doped, or n-type layers.

After growth, a p-contact is formed on the surface of the p-type region.The p-contact 21 often includes multiple conductive layers such as areflective metal and a guard metal which may prevent or reduceelectromigration of the reflective metal. The reflective metal is oftensilver but any suitable material or materials may be used. After formingthe p-contact 21, a portion of the p-contact 21, the p-type region 20,and the active region 18 is removed to expose a portion of the n-typeregion 16 on which an n-contact 22 is formed. The n- and p-contacts 22and 21 are electrically isolated from each other by a gap 25 which maybe filled with a dielectric such as an oxide of silicon or any othersuitable material. Multiple n-contact vias may be formed; the n- andp-contacts 22 and 21 are not limited to the arrangement illustrated inFIG. 2. The n- and p-contacts may be redistributed to form bond padswith a dielectric/metal stack, as is known in the art.

In order to form electrical connections to the LED 1, one or moreinterconnects 26 and 28 are formed on or electrically connected to then- and p-contacts 22 and 21. Interconnect 26 is electrically connectedto n-contact 22 in FIG. 5. Interconnect 28 is electrically connected top-contact 21. Interconnects 26 and 28 are electrically isolated from then- and p-contacts 22 and 21 and from each other by dielectric layer 24and gap 27. Interconnects 26 and 28 may be, for example, solder, studbumps, gold layers, or any other suitable structure.

The substrate 10 may be thinned or entirely removed. In someembodiments, the surface of substrate 10 exposed by thinning ispatterned, textured, or roughened to improve light extraction.

Any suitable light emitting device may be used in light sourcesaccording to embodiments of the invention. The invention is not limitedto the particular LED illustrated in FIG. 2. The light source, such as,for example, the LED illustrated in FIG. 2, is illustrated in thefollowing figures by block 1.

FIGS. 3, 4, and 5 illustrate devices that combine an LED 1 and awavelength converting structure 30. The wavelength converting structuremay contain one of the wavelength converting materials described above.

In FIG. 3, the wavelength converting structure 30 is directly connectedto the LED 1. For example, the wavelength converting structure may bedirectly connected to the substrate 10 illustrated in FIG. 2, or to thesemiconductor structure, if the substrate 10 is removed.

In FIG. 4, the wavelength converting structure 30 is disposed in closeproximity to LED 1, but not directly connected to the LED 1. Forexample, the wavelength converting structure 30 may be separated fromLED 1 by an adhesive layer 32, a small air gap, or any other suitablestructure. The spacing between LED 1 and the wavelength convertingstructure 30 may be, for example, less than 500 μm in some embodiments.

In FIG. 5, the wavelength converting structure 30 is spaced apart fromLED 1. The spacing between LED 1 and the wavelength converting structure30 may be, for example, on the order of millimeters in some embodiments.Such a device may be referred to as a “remote phosphor” device. Remotephosphor arrangements may be used, for example, in backlights fordisplays.

The wavelength converting structure 30 may be square, rectangular,polygonal, hexagonal, circular, or any other suitable shape. Thewavelength converting structure may be the same size as LED 1, largerthan LED 1, or smaller than LED 1.

Wavelength converting structure 30 may be any suitable structure.Wavelength converting structure 30 may be formed separately from LED 1,or formed in situ with LED 1.

Examples of wavelength converting structures that are formed separatelyfrom LED 1 include ceramic wavelength converting structures, that may beformed by sintering or any other suitable process; wavelength convertingmaterials such as powder phosphors that are disposed in transparentmaterial such as silicone or glass that is rolled, cast, or otherwiseformed into a sheet, then singulated into individual wavelengthconverting structures; and wavelength converting materials such aspowder phosphors that are disposed in a transparent material such assilicone that is formed into a flexible sheet, which may be laminated orotherwise disposed over an LED 1.

Examples of wavelength converting structures that are formed in situinclude wavelength converting materials such as powder phosphors thatare mixed with a transparent material such as silicone and dispensed,screen printed, stenciled, molded, or otherwise disposed over LED 1; andwavelength converting materials that are coated on LED 1 byelectrophoretic, vapor, or any other suitable type of deposition.

Multiple forms of wavelength converting structure can be used in asingle device. As just one example, a ceramic wavelength convertingmember can be combined with a molded wavelength converting member, withthe same or different wavelength converting materials in the ceramic andthe molded members.

The wavelength converting structure 30 includes a wavelength convertingmaterial as described above, which may be the only wavelength convertingmaterial in the wavelength converting structure, or one of multiplewavelength converting materials in the wavelength converting structure.The wavelength converting structure 30 may also include, for example,conventional phosphors, organic phosphors, quantum dots, organicsemiconductors, II-VI or III-V semiconductors, II-VI or III-Vsemiconductor quantum dots or nanocrystals, dyes, polymers, or othermaterials that luminesce.

The wavelength converting material absorbs light emitted by the LED andemits light of one or more different wavelengths. Unconverted lightemitted by the LED is often part of the final spectrum of lightextracted from the structure, though it need not be. Examples of commoncombinations include a blue-emitting LED combined with a yellow-emittingwavelength converting material, a blue-emitting LED combined with green-and red-emitting wavelength converting materials, a UV-emitting LEDcombined with blue- and yellow-emitting wavelength converting materials,and a UV-emitting LED combined with blue-, green-, and red-emittingwavelength converting materials. Wavelength converting materialsemitting other colors of light may be added to tailor the spectrum oflight extracted from the structure.

In some embodiments, a wavelength converting material as described aboveis formed into a ceramic, for example by sintering or any suitablemethod. Such a luminescent ceramic may replace garnet-based luminescentceramics due to the expected lower thermal quenching of theabove-described nitride ceramics in, for example, products requiringcool white light such as automotive products. To improve properties ofthe sintered ceramics like light transmission or mechanical strength, asintering step under reduced nitrogen pressure may be followed by anannealing step under increased pressure. The sinterability of theclaimed material can further be enhanced by replacing part of thenitrogen gas atmosphere by hydrogen or helium. For example, in someembodiments, sintering is carried out in a H₂/N₂ 5/95% v/v gas mixture.

Multiple wavelength converting materials may be mixed together or formedas separate structures.

In some embodiments, a blue-emitting LED is combined with ayellow-to-green emitting luminescent ceramic comprising a wavelengthconverting material as described above, and with a red-emittingwavelength converting material. Light from the LED, the luminescentceramic, and the red-emitting wavelength converting material combinesuch that the device emits light that appears white.

In some embodiments, other materials may be added to the wavelengthconverting structure, such as, for example, materials that improveoptical performance, materials that encourage scattering, and/ormaterials that improve thermal performance.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

The invention claimed is:
 1. A device comprising: a wavelength converting material comprising: a cubic crystal structure; an Eu²⁺ dopant; cubic coordination of at least one Eu²⁺ dopant site by X^([2]) (X═N, O) atoms; star-shaped Y(SiX₄)₄ (Y═C, N) host lattice building blocks; AE, where AE=Ca, Sr, Ba; and RE, where RE=Y, Lu, La, Sc.
 2. The device of claim 1 wherein an average effective ionic radii for AE+RE is no more than 120 pm.
 3. The device of claim 1 wherein the wavelength converting material comprises AE_(3−x1−y+z)RE_(3−x2+y−z)[Si_(9−w)Al_(w)(N_(1−y)C_(y))^([4])(N_(16−z−w)O_(z+w))^([2])]:Eu_(x1),Ce_(x2), where AE=Ca, Sr, Ba; RE=Y, Lu, La, Sc; 0≤x1≤0.18; 0≤x2≤0.2; x1+x2>0; 0≤y≤1; 0≤z≤3; 0≤w≤3.
 4. The device of claim 1 further comprising a light emitting diode that emits blue light, wherein the wavelength converting material is disposed in a path of light emitted by the light emitting diode.
 5. The device of claim 1 wherein the wavelength converting material is a first wavelength converting material that emits light having a peak wavelength that is red, the device further comprising a second wavelength converting material that emits light having a peak wavelength that is yellow or green.
 6. The device of claim 1 wherein the wavelength converting material is formed into a ceramic.
 7. The device of claim 6, wherein the wavelength converting material has a density of at least 90% of a density of a single crystal of the wavelength converting material.
 8. The device of claim 1 wherein the wavelength converting material is selected from the group consisting of Ca_(2.49)La_(0.5)Y₃Si₉N_(16.5)C_(0.5):Eu_(0.01), Ca_(2.985)Y₃Si₈AlON₁₆:Eu_(0.015), Ca_(2.985)Y₃Si₉N₁₇:Eu_(0.015), Ca_(1.985)La₄Si₉N₁₆C:Eu_(0.015), Sr_(1.985)Y₄Si₉N₁₆C:Eu_(0.015), Sr_(2.98)Sc₃Si₉N₁₇: Eu_(0.02), Sr_(2.98)Lu₃Si₉N₁₇: Eu_(0.02), and Ca_(5.97)Si₉O₃N₁₄: Eu_(0.03).
 9. The device of claim 1 wherein the wavelength converting material is Ca_(3−x1−y)RE_(3+y)Si₉N_(1−y)C_(y)N₁₆:Eu_(x1) wherein 0.004<=x1<=0.09 and 0<y<=1.
 10. A method comprising: synthesizing a wavelength converting material comprising AE_(3−x1−y+z)RE_(3−x2+y−z)[Si_(9−w)Al_(w)(N_(1−y)C_(y))^([4])(N_(16−z−w)O_(z+w))^([2])]:EU_(x1) Ce_(x2), where AE=Ca, Sr, Ba; RE=Y, Lu, La, Sc; 0≤x1≤0.18; 0≤x2≤0.2; x1+x2>0; 0≤y≤1; 0≤z≤3; 0≤w≤3, said synthesizing comprising: providing a first precursor material comprising an alkaline earth element; providing a second precursor material comprising a rare earth element; providing a silicon source; providing a dopant source; mixing the first precursor material, the second precursor material, the silicon source, and the dopant source; and firing the mixture.
 11. The method of claim 10 wherein the first precursor material is selected from the group consisting of AE₃N₂ and AEH₂, where AE is an alkaline earth element.
 12. The method of claim 10 wherein the second precursor material is selected from the group consisting of silicide RESi₂, silicide Tris[N,N-bis(trimethylsilyl)amide]RE(III), nitride REN, mixed silicides crystallizing in the ThSi₂ structure type, and AE_(1−x)RE_(x)Si₂, where RE is a rare earth element.
 13. The method of claim 10 wherein the silicon source is selected from the group consisting of Si₃N₄, perhydropolysilazane, silicon diimide, silicon, and silicon carbide.
 14. The method of claim 10 wherein the dopant source is selected from the group consisting of Eu₂O₃, Eu₂Si₅N₈, CeO₄, CeF₃, and Tris[N,N-bis(trimethylsilyl)amide]cerium.
 15. The method of claim 10 wherein firing the mixture comprises: forming an intermetallic precursor by a first firing under argon atmosphere; processing the intermetallic precursor under a nitrogen or hydrogen-nitrogen mixture atmosphere to form a nitride phosphor material; and increasing crystallinity of the nitride phosphor material by a second firing under an elevated nitrogen pressure.
 16. The method of claim 10, further comprising providing one or more fluxes, and providing one or more scattering materials. 